The present invention relates generally to EMI filter assemblies, particularly of the type used in active implantable medical devices (AIMDs) such as cardiac pacemakers, cardioverter defibrillators and the like. More particularly, the present invention relates to a filter assembly with increased resistance to arcing/flashover.
Feedthrough terminal pin assemblies are generally well known in the art for use in connecting electrical signals through the housing or case of an electronic instrument. For example, in implantable medical devices such as cardiac pacemakers, defibrillators and the like, the terminal pin assembly comprises one or more conductive terminal pins supported by an insulator structure for feedthrough passage of electrical signals from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known for use in medical devices wherein the insulator structure provides a hermetic seal to prevent entry of patient body fluids into the medical device housing, where such body fluids could otherwise interfere with the operation of and/or cause damage to internal electronic components of the medical device.
In the past, two primary technologies have been employed to manufacture the hermetic seal. One technique involves the use of an alumina insulator which is metallized to accept brazing material. This alumina insulator is brazed to the terminal pin or pins, and also to an outer metal ferrule of titanium or the like. The alumina insulator supports the terminal pin or pins in insulated spaced relation from the ferrule which is adapted for suitable mounting within an access opening formed in the housing of the medical device. In an alternative technique, the hermetic seal comprises a glass-based seal forming a compression or matched fused glass seal for supporting the terminal pin or pins within an outer metal ferrule.
The feedthrough terminal pins are typically connected to one or more leadwires which, in the example of a cardiac pacemaker, sense signals from the patient's heart and also couple electronic pacing pulses from the medical device to the patient's heart. Unfortunately, these leadwires can act as an antenna to collect stray electromagnetic interference (EMI) signals for transmission via the terminal pins into the interior of the medical device. Such unwanted EMI signals can disrupt proper operation of the medical device, resulting in malfunction or failure. For example, it has been documented that stray EMI signals emanating from cellular telephones can inhibit pacemaker operation, resulting in asynchronous pacing, tracking and missed beats. To address this problem, hermetically sealed feedthrough terminal pin assemblies have been designed to include a feedthrough capacitor for decoupling EMI signals in a manner preventing such unwanted signals from entering the housing of the implantable medical device. See, for example, U.S. Pat. Nos. 4,424,551; 5,333,095; 5,751,539; 5,905,627; 5,973,906; 6,008,980; and 6,566,978. These prior art feedthrough capacitor EMI filters generally provide a high degree of attenuation to EMI in the frequency range between 450 and 3000 MHz.
Accordingly, there is a need for circuit protection devices which will improve the immunity of active implantable medical device systems to diagnostic procedures such as MRI. There is also a need to provide increased filtering for AIMD's due to the recent proliferation in the marketplace of new higher power emitters. These include aftermarket cellular telephone amplifiers, associated higher gain antennas and radio frequency indentification (RFID) readers and scanners. The present invention fulfills all of these needs and provides other related advantages.